Automotive metal fasteners are usually coated or plated to enhance various characteristics such as resistance to corrosion, resistance to seizing/galling, low fastening friction, economy, solderability, and resistance to the stick-slip phenomenon (which is a repeated sticking followed by repeated slipping during fastener tightening operations).
To provide resistance to corrosion, it has become conventional in the automotive fastener art to coat the fastener with phosphate and oil or to electroplate with zinc. Unfortunately such coated or plated fasteners are subject to the stick-slip phenomenon when the fastener is tightened with power tools, causing the desired torque loading to sometimes be off target. This stick-slip phenomenon is in part related to a relatively high coefficient of friction for the coating or plating. As the power tool turns the coated or plated nut onto a threaded bolt, the phosphate or zinc is spalled off and collects in small masses to form miniature wedges between the threads. The torque tool will shut off prematurely in response to the effect of these wedges, leaving the mechanical assembly improperly secured under such circumstances, which assembly must later be reworked when identified by an inspecting system.
To obviate the stick-slip problem and to impart a lower coefficient of friction to the coating or plating, cadmium has been employed to impart lubricity and good sacrificial corrosion protection, particularly in a marine environment (see Modern Electroplating, by F. A. Lowenheim, published by John Wiley & Sons, 3rd Edition, p. 663, 1974). However, cadium is subject to two disadvantages: (a) it has a toxic effect during processing, and (b) it is significantly expensive.
A codeposited material which would combine the low coefficient of friction of each of its constituent ingredients would appear to offer potential in meeting the various characteristics desired for an automotive fastener; but this has not necessarily proven to be the case. Experimentation with codeposits of metal and inorganic particles has been reported in the literature.
In an article entitled "Codeposition of Finely Dispersed Inorganic Particles" by Tomaszewski et al, published in Plating, Vol. 56, p. 1234, November 1969, the author investigated using acidic copper plating and acid nickel baths containing various inorganic particles including graphite, MoS.sub.2, BaSO.sub.4, SrSO.sub.4, Al.sub.2 O.sub.3, TiO.sub.2, ZrO.sub.2, H.sub.2 Al.sub.2 Si.sub.2 O.sub.8.H.sub.2 O, PbSO.sub.4, Pb.sub.3 (PO.sub.4).sub.2, CeO.sub.2, BN, B.sub.6 C, B, Si, and SiO.sub.2.
Some of these codeposited particles can be considered nonconductive and normally would not respond to the normal electrolytic action, but it was found that even graphite would plate or codeposit under very strained and undesirable conditions with nickel. The metal matrix and codeposited particles were viewed as to their antifriction, antiseizing, and dry lubrication properties and found them not lower than zinc or cadmium. No investigation was made of the mode of corrosion of such codeposits. Without exploring proper processing parameters, the author concluded that codeposition was feasible only at conventional metal plating parameters. Similar observations were made by Parker as to electroless nickel deposits, entitled "Hardness and Wear Resistance Tests of Electroless Nickel Deposits", Journal of Plating, Vol. 61, p. 834, September 1974. None of the codeposits investigated by these scientists provided a low enough coefficient of friction which would be comparable to cadmium plating now used in the art. Thus these coatings were lacking in a good antigalling characteristic and good corrosion resistance. Nickel does not fail by a sacrifical corrosion mode and thus requires inordinately thick deposits to act as a physical barrier and protect the substrate. The low coefficient of friction of each of the constituent ingredients did not necessarily add up to a total lower coefficient of friction for the codeposited material. In addition, each codeposit lacked good cathode efficiency and low material cost.
One group of investigators, Messrs. Ghouse and Ramachandren, explored the "Antifriction Properties of Electrodeposited Composites of Graphite or Molybdenum Disulfide With Copper", Journal of Metal Finishing, Vol. 78, p. 85, June 1981. The amount of second phase material (graphite or MoS.sub.2) was found to have a slight effect on total coefficient of friction of the combined material but insufficient to make the codeposit have an improved coefficient of friction comparable to cadmium.
Since nickel by itself is much lower in its coefficient of friction than zinc, and since the codeposit of nickel and graphite fails to provide a significantly lower coefficient of friction for the codeposited material than nickel or graphite, the state of the art must draw or extrapolate from these conclusions that zinc and graphite would be no better in such characteristics than zinc by itself and would have a higher coefficient of friction than nickel.
What is needed by the prior art is a codeposited material and accompanying method of depositing such material, which material is equal to or better than cadmium plating with respect to the coefficient of friction, has no toxic effects, is superior in antiseizing and antigalling characteristics, is high in cathode efficiency, offers reasonably good corrosion resistance in a nonmarine environment, and is solderable.